/**
 * @file
 * DNS - host name to IP address resolver.
 *
 * @defgroup dns DNS
 * @ingroup callbackstyle_api
 *
 * Implements a DNS host name to IP address resolver.
 *
 * The lwIP DNS resolver functions are used to lookup a host name and
 * map it to a numerical IP address. It maintains a list of resolved
 * hostnames that can be queried with the dns_lookup() function.
 * New hostnames can be resolved using the dns_query() function.
 *
 * The lwIP version of the resolver also adds a non-blocking version of
 * gethostbyname() that will work with a raw API application. This function
 * checks for an IP address string first and converts it if it is valid.
 * gethostbyname() then does a dns_lookup() to see if the name is
 * already in the table. If so, the IP is returned. If not, a query is
 * issued and the function returns with a ERR_INPROGRESS status. The app
 * using the dns client must then go into a waiting state.
 *
 * Once a hostname has been resolved (or found to be non-existent),
 * the resolver code calls a specified callback function (which
 * must be implemented by the module that uses the resolver).
 *
 * Multicast DNS queries are supported for names ending on ".local".
 * However, only "One-Shot Multicast DNS Queries" are supported (RFC 6762
 * chapter 5.1), this is not a fully compliant implementation of continuous
 * mDNS querying!
 *
 * All functions must be called from TCPIP thread.
 *
 * @see @ref netconn_common for thread-safe access.
 */

/*
 * Port to lwIP from uIP
 * by Jim Pettinato April 2007
 *
 * security fixes and more by Simon Goldschmidt
 *
 * uIP version Copyright (c) 2002-2003, Adam Dunkels.
 * All rights reserved.
 *
 * Redistribution and use in source and binary forms, with or without
 * modification, are permitted provided that the following conditions
 * are met:
 * 1. Redistributions of source code must retain the above copyright
 *    notice, this list of conditions and the following disclaimer.
 * 2. Redistributions in binary form must reproduce the above copyright
 *    notice, this list of conditions and the following disclaimer in the
 *    documentation and/or other materials provided with the distribution.
 * 3. The name of the author may not be used to endorse or promote
 *    products derived from this software without specific prior
 *    written permission.
 *
 * THIS SOFTWARE IS PROVIDED BY THE AUTHOR ``AS IS'' AND ANY EXPRESS
 * OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED
 * WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
 * ARE DISCLAIMED.  IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY
 * DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
 * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE
 * GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS
 * INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY,
 * WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING
 * NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS
 * SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 */

/*-----------------------------------------------------------------------------
 * RFC 1035 - Domain names - implementation and specification
 * RFC 2181 - Clarifications to the DNS Specification
 *----------------------------------------------------------------------------*/

/** @todo: define good default values (rfc compliance) */
/** @todo: improve answer parsing, more checkings... */
/** @todo: check RFC1035 - 7.3. Processing responses */
/** @todo: one-shot mDNS: dual-stack fallback to another IP version */

/*-----------------------------------------------------------------------------
 * Includes
 *----------------------------------------------------------------------------*/

#include "lwip/opt.h"

#if LWIP_DNS /* don't build if not configured for use in lwipopts.h */

#include "lwip/def.h"
#include "lwip/udp.h"
#include "lwip/mem.h"
#include "lwip/memp.h"
#include "lwip/dns.h"
#include "lwip/prot/dns.h"

#include <string.h>

/** Random generator function to create random TXIDs and source ports for queries */
#ifndef DNS_RAND_TXID
	#if ((LWIP_DNS_SECURE & LWIP_DNS_SECURE_RAND_XID) != 0)
		#define DNS_RAND_TXID LWIP_RAND
	#else
		static u16_t dns_txid;
		#define DNS_RAND_TXID() (++dns_txid)
	#endif
#endif

/** Limits the source port to be >= 1024 by default */
#ifndef DNS_PORT_ALLOWED
	#define DNS_PORT_ALLOWED(port) ((port) >= 1024)
#endif

/** DNS maximum number of retries when asking for a name, before "timeout". */
#ifndef DNS_MAX_RETRIES
	#define DNS_MAX_RETRIES           4
#endif

/** DNS resource record max. TTL (one week as default) */
#ifndef DNS_MAX_TTL
	#define DNS_MAX_TTL               604800
#elif DNS_MAX_TTL > 0x7FFFFFFF
	#error DNS_MAX_TTL must be a positive 32-bit value
#endif

#if DNS_TABLE_SIZE > 255
	#error DNS_TABLE_SIZE must fit into an u8_t
#endif
#if DNS_MAX_SERVERS > 255
	#error DNS_MAX_SERVERS must fit into an u8_t
#endif

/* The number of parallel requests (i.e. calls to dns_gethostbyname
 * that cannot be answered from the DNS table.
 * This is set to the table size by default.
 */
#if ((LWIP_DNS_SECURE & LWIP_DNS_SECURE_NO_MULTIPLE_OUTSTANDING) != 0)
	#ifndef DNS_MAX_REQUESTS
		#define DNS_MAX_REQUESTS          DNS_TABLE_SIZE
	#else
		#if DNS_MAX_REQUESTS > 255
			#error DNS_MAX_REQUESTS must fit into an u8_t
		#endif
	#endif
#else
	/* In this configuration, both arrays have to have the same size and are used
	* like one entry (used/free) */
	#define DNS_MAX_REQUESTS          DNS_TABLE_SIZE
#endif

/* The number of UDP source ports used in parallel */
#if ((LWIP_DNS_SECURE & LWIP_DNS_SECURE_RAND_SRC_PORT) != 0)
	#ifndef DNS_MAX_SOURCE_PORTS
		#define DNS_MAX_SOURCE_PORTS      DNS_MAX_REQUESTS
	#else
		#if DNS_MAX_SOURCE_PORTS > 255
			#error DNS_MAX_SOURCE_PORTS must fit into an u8_t
		#endif
	#endif
#else
	#ifdef DNS_MAX_SOURCE_PORTS
		#undef DNS_MAX_SOURCE_PORTS
	#endif
	#define DNS_MAX_SOURCE_PORTS      1
#endif

#if LWIP_IPV4 && LWIP_IPV6
	#define LWIP_DNS_ADDRTYPE_IS_IPV6(t) (((t) == LWIP_DNS_ADDRTYPE_IPV6_IPV4) || ((t) == LWIP_DNS_ADDRTYPE_IPV6))
	#define LWIP_DNS_ADDRTYPE_MATCH_IP(t, ip) (IP_IS_V6_VAL(ip) ? LWIP_DNS_ADDRTYPE_IS_IPV6(t) : (!LWIP_DNS_ADDRTYPE_IS_IPV6(t)))
	#define LWIP_DNS_ADDRTYPE_ARG(x) , x
	#define LWIP_DNS_ADDRTYPE_ARG_OR_ZERO(x) x
	#define LWIP_DNS_SET_ADDRTYPE(x, y) do { x = y; } while(0)
#else
	#if LWIP_IPV6
		#define LWIP_DNS_ADDRTYPE_IS_IPV6(t) 1
	#else
		#define LWIP_DNS_ADDRTYPE_IS_IPV6(t) 0
	#endif
	#define LWIP_DNS_ADDRTYPE_MATCH_IP(t, ip) 1
	#define LWIP_DNS_ADDRTYPE_ARG(x)
	#define LWIP_DNS_ADDRTYPE_ARG_OR_ZERO(x) 0
	#define LWIP_DNS_SET_ADDRTYPE(x, y)
#endif /* LWIP_IPV4 && LWIP_IPV6 */

#if LWIP_DNS_SUPPORT_MDNS_QUERIES
	#define LWIP_DNS_ISMDNS_ARG(x) , x
#else
	#define LWIP_DNS_ISMDNS_ARG(x)
#endif

/** DNS query message structure.
    No packing needed: only used locally on the stack. */
struct dns_query {
	/* DNS query record starts with either a domain name or a pointer
	   to a name already present somewhere in the packet. */
	u16_t type;
	u16_t cls;
};
#define SIZEOF_DNS_QUERY 4

/** DNS answer message structure.
    No packing needed: only used locally on the stack. */
struct dns_answer {
	/* DNS answer record starts with either a domain name or a pointer
	   to a name already present somewhere in the packet. */
	u16_t type;
	u16_t cls;
	u32_t ttl;
	u16_t len;
};
#define SIZEOF_DNS_ANSWER 10
/* maximum allowed size for the struct due to non-packed */
#define SIZEOF_DNS_ANSWER_ASSERT 12

/* DNS table entry states */
typedef enum {
	DNS_STATE_UNUSED           = 0,
	DNS_STATE_NEW              = 1,
	DNS_STATE_ASKING           = 2,
	DNS_STATE_DONE             = 3
} dns_state_enum_t;

/** DNS table entry */
struct dns_table_entry {
	u32_t ttl;
	ip_addr_t ipaddr;
	u16_t txid;
	u8_t  state;
	u8_t  server_idx;
	u8_t  tmr;
	u8_t  retries;
	u8_t  seqno;
#if ((LWIP_DNS_SECURE & LWIP_DNS_SECURE_RAND_SRC_PORT) != 0)
	u8_t pcb_idx;
#endif
	char name[DNS_MAX_NAME_LENGTH];
#if LWIP_IPV4 && LWIP_IPV6
	u8_t reqaddrtype;
#endif /* LWIP_IPV4 && LWIP_IPV6 */
#if LWIP_DNS_SUPPORT_MDNS_QUERIES
	u8_t is_mdns;
#endif
};

/** DNS request table entry: used when dns_gehostbyname cannot answer the
 * request from the DNS table */
struct dns_req_entry {
	/* pointer to callback on DNS query done */
	dns_found_callback found;
	/* argument passed to the callback function */
	void* arg;
#if ((LWIP_DNS_SECURE & LWIP_DNS_SECURE_NO_MULTIPLE_OUTSTANDING) != 0)
	u8_t dns_table_idx;
#endif
#if LWIP_IPV4 && LWIP_IPV6
	u8_t reqaddrtype;
#endif /* LWIP_IPV4 && LWIP_IPV6 */
};

#if DNS_LOCAL_HOSTLIST

	#if DNS_LOCAL_HOSTLIST_IS_DYNAMIC
		/** Local host-list. For hostnames in this list, no
		*  external name resolution is performed */
		static struct local_hostlist_entry* local_hostlist_dynamic;
	#else /* DNS_LOCAL_HOSTLIST_IS_DYNAMIC */

		/** Defining this allows the local_hostlist_static to be placed in a different
		* linker section (e.g. FLASH) */
		#ifndef DNS_LOCAL_HOSTLIST_STORAGE_PRE
			#define DNS_LOCAL_HOSTLIST_STORAGE_PRE static
		#endif /* DNS_LOCAL_HOSTLIST_STORAGE_PRE */
		/** Defining this allows the local_hostlist_static to be placed in a different
		* linker section (e.g. FLASH) */
		#ifndef DNS_LOCAL_HOSTLIST_STORAGE_POST
			#define DNS_LOCAL_HOSTLIST_STORAGE_POST
		#endif /* DNS_LOCAL_HOSTLIST_STORAGE_POST */
		DNS_LOCAL_HOSTLIST_STORAGE_PRE struct local_hostlist_entry local_hostlist_static[]
		DNS_LOCAL_HOSTLIST_STORAGE_POST = DNS_LOCAL_HOSTLIST_INIT;

	#endif /* DNS_LOCAL_HOSTLIST_IS_DYNAMIC */

	static void dns_init_local(void);
	static err_t dns_lookup_local(const char* hostname, ip_addr_t* addr LWIP_DNS_ADDRTYPE_ARG(u8_t dns_addrtype));
#endif /* DNS_LOCAL_HOSTLIST */


/* forward declarations */
static void dns_recv(void* s, struct udp_pcb* pcb, struct pbuf* p, const ip_addr_t* addr, u16_t port);
static void dns_check_entries(void);
static void dns_call_found(u8_t idx, ip_addr_t* addr);

/*-----------------------------------------------------------------------------
 * Globals
 *----------------------------------------------------------------------------*/

/* DNS variables */
static struct udp_pcb*        dns_pcbs[DNS_MAX_SOURCE_PORTS];
#if ((LWIP_DNS_SECURE & LWIP_DNS_SECURE_RAND_SRC_PORT) != 0)
	static u8_t                   dns_last_pcb_idx;
#endif
static u8_t                   dns_seqno;
static struct dns_table_entry dns_table[DNS_TABLE_SIZE];
static struct dns_req_entry   dns_requests[DNS_MAX_REQUESTS];
static ip_addr_t              dns_servers[DNS_MAX_SERVERS];

#if LWIP_IPV4
	const ip_addr_t dns_mquery_v4group = DNS_MQUERY_IPV4_GROUP_INIT;
#endif /* LWIP_IPV4 */
#if LWIP_IPV6
	const ip_addr_t dns_mquery_v6group = DNS_MQUERY_IPV6_GROUP_INIT;
#endif /* LWIP_IPV6 */

/**
 * Initialize the resolver: set up the UDP pcb and configure the default server
 * (if DNS_SERVER_ADDRESS is set).
 */
void
dns_init(void)
{
#ifdef DNS_SERVER_ADDRESS
	/* initialize default DNS server address */
	ip_addr_t dnsserver;
	DNS_SERVER_ADDRESS(&dnsserver);
	dns_setserver(0, &dnsserver);
#endif /* DNS_SERVER_ADDRESS */

	LWIP_ASSERT("sanity check SIZEOF_DNS_QUERY",
	            sizeof(struct dns_query) == SIZEOF_DNS_QUERY);
	LWIP_ASSERT("sanity check SIZEOF_DNS_ANSWER",
	            sizeof(struct dns_answer) <= SIZEOF_DNS_ANSWER_ASSERT);

	LWIP_DEBUGF(DNS_DEBUG, ("dns_init: initializing\n"));

	/* if dns client not yet initialized... */
#if ((LWIP_DNS_SECURE & LWIP_DNS_SECURE_RAND_SRC_PORT) == 0)

	if(dns_pcbs[0] == NULL) {
		dns_pcbs[0] = udp_new_ip_type(IPADDR_TYPE_ANY);
		LWIP_ASSERT("dns_pcbs[0] != NULL", dns_pcbs[0] != NULL);

		/* initialize DNS table not needed (initialized to zero since it is a
		 * global variable) */
		LWIP_ASSERT("For implicit initialization to work, DNS_STATE_UNUSED needs to be 0",
		            DNS_STATE_UNUSED == 0);

		/* initialize DNS client */
		udp_bind(dns_pcbs[0], IP_ANY_TYPE, 0);
		udp_recv(dns_pcbs[0], dns_recv, NULL);
	}

#endif

#if DNS_LOCAL_HOSTLIST
	dns_init_local();
#endif
}

/**
 * @ingroup dns
 * Initialize one of the DNS servers.
 *
 * @param numdns the index of the DNS server to set must be < DNS_MAX_SERVERS
 * @param dnsserver IP address of the DNS server to set
 */
void
dns_setserver(u8_t numdns, const ip_addr_t* dnsserver)
{
	if(numdns < DNS_MAX_SERVERS) {
		if(dnsserver != NULL) {
			dns_servers[numdns] = (*dnsserver);
		} else {
			dns_servers[numdns] = *IP_ADDR_ANY;
		}
	}
}

/**
 * @ingroup dns
 * Obtain one of the currently configured DNS server.
 *
 * @param numdns the index of the DNS server
 * @return IP address of the indexed DNS server or "ip_addr_any" if the DNS
 *         server has not been configured.
 */
const ip_addr_t*
dns_getserver(u8_t numdns)
{
	if(numdns < DNS_MAX_SERVERS) {
		return &dns_servers[numdns];
	} else {
		return IP_ADDR_ANY;
	}
}

/**
 * The DNS resolver client timer - handle retries and timeouts and should
 * be called every DNS_TMR_INTERVAL milliseconds (every second by default).
 */
void
dns_tmr(void)
{
	LWIP_DEBUGF(DNS_DEBUG, ("dns_tmr: dns_check_entries\n"));
	dns_check_entries();
}

#if DNS_LOCAL_HOSTLIST
static void
dns_init_local(void)
{
#if DNS_LOCAL_HOSTLIST_IS_DYNAMIC && defined(DNS_LOCAL_HOSTLIST_INIT)
	size_t i;
	struct local_hostlist_entry* entry;
	/* Dynamic: copy entries from DNS_LOCAL_HOSTLIST_INIT to list */
	struct local_hostlist_entry local_hostlist_init[] = DNS_LOCAL_HOSTLIST_INIT;
	size_t namelen;

	for(i = 0; i < LWIP_ARRAYSIZE(local_hostlist_init); i++) {
		struct local_hostlist_entry* init_entry = &local_hostlist_init[i];
		LWIP_ASSERT("invalid host name (NULL)", init_entry->name != NULL);
		namelen = strlen(init_entry->name);
		LWIP_ASSERT("namelen <= DNS_LOCAL_HOSTLIST_MAX_NAMELEN", namelen <= DNS_LOCAL_HOSTLIST_MAX_NAMELEN);
		entry = (struct local_hostlist_entry*)memp_malloc(MEMP_LOCALHOSTLIST);
		LWIP_ASSERT("mem-error in dns_init_local", entry != NULL);

		if(entry != NULL) {
			char* entry_name = (char*)entry + sizeof(struct local_hostlist_entry);
			MEMCPY(entry_name, init_entry->name, namelen);
			entry_name[namelen] = 0;
			entry->name = entry_name;
			entry->addr = init_entry->addr;
			entry->next = local_hostlist_dynamic;
			local_hostlist_dynamic = entry;
		}
	}

#endif /* DNS_LOCAL_HOSTLIST_IS_DYNAMIC && defined(DNS_LOCAL_HOSTLIST_INIT) */
}

/**
 * @ingroup dns
 * Iterate the local host-list for a hostname.
 *
 * @param iterator_fn a function that is called for every entry in the local host-list
 * @param iterator_arg 3rd argument passed to iterator_fn
 * @return the number of entries in the local host-list
 */
size_t
dns_local_iterate(dns_found_callback iterator_fn, void* iterator_arg)
{
	size_t i;
#if DNS_LOCAL_HOSTLIST_IS_DYNAMIC
	struct local_hostlist_entry* entry = local_hostlist_dynamic;
	i = 0;

	while(entry != NULL) {
		if(iterator_fn != NULL) {
			iterator_fn(entry->name, &entry->addr, iterator_arg);
		}

		i++;
		entry = entry->next;
	}

#else /* DNS_LOCAL_HOSTLIST_IS_DYNAMIC */

	for(i = 0; i < LWIP_ARRAYSIZE(local_hostlist_static); i++) {
		if(iterator_fn != NULL) {
			iterator_fn(local_hostlist_static[i].name, &local_hostlist_static[i].addr, iterator_arg);
		}
	}

#endif /* DNS_LOCAL_HOSTLIST_IS_DYNAMIC */
	return i;
}

/**
 * @ingroup dns
 * Scans the local host-list for a hostname.
 *
 * @param hostname Hostname to look for in the local host-list
 * @param addr the first IP address for the hostname in the local host-list or
 *         IPADDR_NONE if not found.
 * @param dns_addrtype - LWIP_DNS_ADDRTYPE_IPV4_IPV6: try to resolve IPv4 (ATTENTION: no fallback here!)
 *                     - LWIP_DNS_ADDRTYPE_IPV6_IPV4: try to resolve IPv6 (ATTENTION: no fallback here!)
 *                     - LWIP_DNS_ADDRTYPE_IPV4: try to resolve IPv4 only
 *                     - LWIP_DNS_ADDRTYPE_IPV6: try to resolve IPv6 only
 * @return ERR_OK if found, ERR_ARG if not found
 */
err_t
dns_local_lookup(const char* hostname, ip_addr_t* addr, u8_t dns_addrtype)
{
	LWIP_UNUSED_ARG(dns_addrtype);
	return dns_lookup_local(hostname, addr LWIP_DNS_ADDRTYPE_ARG(dns_addrtype));
}

/* Internal implementation for dns_local_lookup and dns_lookup */
static err_t
dns_lookup_local(const char* hostname, ip_addr_t* addr LWIP_DNS_ADDRTYPE_ARG(u8_t dns_addrtype))
{
#if DNS_LOCAL_HOSTLIST_IS_DYNAMIC
	struct local_hostlist_entry* entry = local_hostlist_dynamic;

	while(entry != NULL) {
		if((lwip_stricmp(entry->name, hostname) == 0) &&
		        LWIP_DNS_ADDRTYPE_MATCH_IP(dns_addrtype, entry->addr)) {
			if(addr) {
				ip_addr_copy(*addr, entry->addr);
			}

			return ERR_OK;
		}

		entry = entry->next;
	}

#else /* DNS_LOCAL_HOSTLIST_IS_DYNAMIC */
	size_t i;

	for(i = 0; i < LWIP_ARRAYSIZE(local_hostlist_static); i++) {
		if((lwip_stricmp(local_hostlist_static[i].name, hostname) == 0) &&
		        LWIP_DNS_ADDRTYPE_MATCH_IP(dns_addrtype, local_hostlist_static[i].addr)) {
			if(addr) {
				ip_addr_copy(*addr, local_hostlist_static[i].addr);
			}

			return ERR_OK;
		}
	}

#endif /* DNS_LOCAL_HOSTLIST_IS_DYNAMIC */
	return ERR_ARG;
}

#if DNS_LOCAL_HOSTLIST_IS_DYNAMIC
/**
 * @ingroup dns
 * Remove all entries from the local host-list for a specific hostname
 * and/or IP address
 *
 * @param hostname hostname for which entries shall be removed from the local
 *                 host-list
 * @param addr address for which entries shall be removed from the local host-list
 * @return the number of removed entries
 */
int
dns_local_removehost(const char* hostname, const ip_addr_t* addr)
{
	int removed = 0;
	struct local_hostlist_entry* entry = local_hostlist_dynamic;
	struct local_hostlist_entry* last_entry = NULL;

	while(entry != NULL) {
		if(((hostname == NULL) || !lwip_stricmp(entry->name, hostname)) &&
		        ((addr == NULL) || ip_addr_cmp(&entry->addr, addr))) {
			struct local_hostlist_entry* free_entry;

			if(last_entry != NULL) {
				last_entry->next = entry->next;
			} else {
				local_hostlist_dynamic = entry->next;
			}

			free_entry = entry;
			entry = entry->next;
			memp_free(MEMP_LOCALHOSTLIST, free_entry);
			removed++;
		} else {
			last_entry = entry;
			entry = entry->next;
		}
	}

	return removed;
}

/**
 * @ingroup dns
 * Add a hostname/IP address pair to the local host-list.
 * Duplicates are not checked.
 *
 * @param hostname hostname of the new entry
 * @param addr IP address of the new entry
 * @return ERR_OK if succeeded or ERR_MEM on memory error
 */
err_t
dns_local_addhost(const char* hostname, const ip_addr_t* addr)
{
	struct local_hostlist_entry* entry;
	size_t namelen;
	char* entry_name;
	LWIP_ASSERT("invalid host name (NULL)", hostname != NULL);
	namelen = strlen(hostname);
	LWIP_ASSERT("namelen <= DNS_LOCAL_HOSTLIST_MAX_NAMELEN", namelen <= DNS_LOCAL_HOSTLIST_MAX_NAMELEN);
	entry = (struct local_hostlist_entry*)memp_malloc(MEMP_LOCALHOSTLIST);

	if(entry == NULL) {
		return ERR_MEM;
	}

	entry_name = (char*)entry + sizeof(struct local_hostlist_entry);
	MEMCPY(entry_name, hostname, namelen);
	entry_name[namelen] = 0;
	entry->name = entry_name;
	ip_addr_copy(entry->addr, *addr);
	entry->next = local_hostlist_dynamic;
	local_hostlist_dynamic = entry;
	return ERR_OK;
}
#endif /* DNS_LOCAL_HOSTLIST_IS_DYNAMIC*/
#endif /* DNS_LOCAL_HOSTLIST */

/**
 * @ingroup dns
 * Look up a hostname in the array of known hostnames.
 *
 * @note This function only looks in the internal array of known
 * hostnames, it does not send out a query for the hostname if none
 * was found. The function dns_enqueue() can be used to send a query
 * for a hostname.
 *
 * @param name the hostname to look up
 * @param addr the hostname's IP address, as u32_t (instead of ip_addr_t to
 *         better check for failure: != IPADDR_NONE) or IPADDR_NONE if the hostname
 *         was not found in the cached dns_table.
 * @return ERR_OK if found, ERR_ARG if not found
 */
static err_t
dns_lookup(const char* name, ip_addr_t* addr LWIP_DNS_ADDRTYPE_ARG(u8_t dns_addrtype))
{
	u8_t i;
#if DNS_LOCAL_HOSTLIST || defined(DNS_LOOKUP_LOCAL_EXTERN)
#endif /* DNS_LOCAL_HOSTLIST || defined(DNS_LOOKUP_LOCAL_EXTERN) */
#if DNS_LOCAL_HOSTLIST

	if(dns_lookup_local(name, addr LWIP_DNS_ADDRTYPE_ARG(dns_addrtype)) == ERR_OK) {
		return ERR_OK;
	}

#endif /* DNS_LOCAL_HOSTLIST */
#ifdef DNS_LOOKUP_LOCAL_EXTERN

	if(DNS_LOOKUP_LOCAL_EXTERN(name, addr, LWIP_DNS_ADDRTYPE_ARG_OR_ZERO(dns_addrtype)) == ERR_OK) {
		return ERR_OK;
	}

#endif /* DNS_LOOKUP_LOCAL_EXTERN */

	/* Walk through name list, return entry if found. If not, return NULL. */
	for(i = 0; i < DNS_TABLE_SIZE; ++i) {
		if((dns_table[i].state == DNS_STATE_DONE) &&
		        (lwip_strnicmp(name, dns_table[i].name, sizeof(dns_table[i].name)) == 0) &&
		        LWIP_DNS_ADDRTYPE_MATCH_IP(dns_addrtype, dns_table[i].ipaddr)) {
			LWIP_DEBUGF(DNS_DEBUG, ("dns_lookup: \"%s\": found = ", name));
			ip_addr_debug_print(DNS_DEBUG, &(dns_table[i].ipaddr));
			LWIP_DEBUGF(DNS_DEBUG, ("\n"));

			if(addr) {
				ip_addr_copy(*addr, dns_table[i].ipaddr);
			}

			return ERR_OK;
		}
	}

	return ERR_ARG;
}

/**
 * Compare the "dotted" name "query" with the encoded name "response"
 * to make sure an answer from the DNS server matches the current dns_table
 * entry (otherwise, answers might arrive late for hostname not on the list
 * any more).
 *
 * @param query hostname (not encoded) from the dns_table
 * @param p pbuf containing the encoded hostname in the DNS response
 * @param start_offset offset into p where the name starts
 * @return 0xFFFF: names differ, other: names equal -> offset behind name
 */
static u16_t
dns_compare_name(const char* query, struct pbuf* p, u16_t start_offset)
{
	int n;
	u16_t response_offset = start_offset;

	do {
		n = pbuf_try_get_at(p, response_offset++);

		if(n < 0) {
			return 0xFFFF;
		}

		/** @see RFC 1035 - 4.1.4. Message compression */
		if((n & 0xc0) == 0xc0) {
			/* Compressed name: cannot be equal since we don't send them */
			return 0xFFFF;
		} else {
			/* Not compressed name */
			while(n > 0) {
				int c = pbuf_try_get_at(p, response_offset);

				if(c < 0) {
					return 0xFFFF;
				}

				if((*query) != (u8_t)c) {
					return 0xFFFF;
				}

				++response_offset;
				++query;
				--n;
			}

			++query;
		}

		n = pbuf_try_get_at(p, response_offset);

		if(n < 0) {
			return 0xFFFF;
		}
	} while(n != 0);

	return response_offset + 1;
}

/**
 * Walk through a compact encoded DNS name and return the end of the name.
 *
 * @param p pbuf containing the name
 * @param query_idx start index into p pointing to encoded DNS name in the DNS server response
 * @return index to end of the name
 */
static u16_t
dns_skip_name(struct pbuf* p, u16_t query_idx)
{
	int n;
	u16_t offset = query_idx;

	do {
		n = pbuf_try_get_at(p, offset++);

		if(n < 0) {
			return 0xFFFF;
		}

		/** @see RFC 1035 - 4.1.4. Message compression */
		if((n & 0xc0) == 0xc0) {
			/* Compressed name: since we only want to skip it (not check it), stop here */
			break;
		} else {
			/* Not compressed name */
			if(offset + n >= p->tot_len) {
				return 0xFFFF;
			}

			offset = (u16_t)(offset + n);
		}

		n = pbuf_try_get_at(p, offset);

		if(n < 0) {
			return 0xFFFF;
		}
	} while(n != 0);

	return offset + 1;
}

/**
 * Send a DNS query packet.
 *
 * @param idx the DNS table entry index for which to send a request
 * @return ERR_OK if packet is sent; an err_t indicating the problem otherwise
 */
static err_t
dns_send(u8_t idx)
{
	err_t err;
	struct dns_hdr hdr;
	struct dns_query qry;
	struct pbuf* p;
	u16_t query_idx, copy_len;
	const char* hostname, *hostname_part;
	u8_t n;
	u8_t pcb_idx;
	struct dns_table_entry* entry = &dns_table[idx];

	LWIP_DEBUGF(DNS_DEBUG, ("dns_send: dns_servers[%"U16_F"] \"%s\": request\n",
	                        (u16_t)(entry->server_idx), entry->name));
	LWIP_ASSERT("dns server out of array", entry->server_idx < DNS_MAX_SERVERS);

	if(ip_addr_isany_val(dns_servers[entry->server_idx])
#if LWIP_DNS_SUPPORT_MDNS_QUERIES
	        && !entry->is_mdns
#endif
	  ) {
		/* DNS server not valid anymore, e.g. PPP netif has been shut down */
		/* call specified callback function if provided */
		dns_call_found(idx, NULL);
		/* flush this entry */
		entry->state = DNS_STATE_UNUSED;
		return ERR_OK;
	}

	/* if here, we have either a new query or a retry on a previous query to process */
	p = pbuf_alloc(PBUF_TRANSPORT, (u16_t)(SIZEOF_DNS_HDR + strlen(entry->name) + 2 +
	                                       SIZEOF_DNS_QUERY), PBUF_RAM);

	if(p != NULL) {
		const ip_addr_t* dst;
		u16_t dst_port;
		/* fill dns header */
		memset(&hdr, 0, SIZEOF_DNS_HDR);
		hdr.id = lwip_htons(entry->txid);
		hdr.flags1 = DNS_FLAG1_RD;
		hdr.numquestions = PP_HTONS(1);
		pbuf_take(p, &hdr, SIZEOF_DNS_HDR);
		hostname = entry->name;
		--hostname;

		/* convert hostname into suitable query format. */
		query_idx = SIZEOF_DNS_HDR;

		do {
			++hostname;
			hostname_part = hostname;

			for(n = 0; *hostname != '.' && *hostname != 0; ++hostname) {
				++n;
			}

			copy_len = (u16_t)(hostname - hostname_part);
			pbuf_put_at(p, query_idx, n);
			pbuf_take_at(p, hostname_part, copy_len, query_idx + 1);
			query_idx += n + 1;
		} while(*hostname != 0);

		pbuf_put_at(p, query_idx, 0);
		query_idx++;

		/* fill dns query */
		if(LWIP_DNS_ADDRTYPE_IS_IPV6(entry->reqaddrtype)) {
			qry.type = PP_HTONS(DNS_RRTYPE_AAAA);
		} else {
			qry.type = PP_HTONS(DNS_RRTYPE_A);
		}

		qry.cls = PP_HTONS(DNS_RRCLASS_IN);
		pbuf_take_at(p, &qry, SIZEOF_DNS_QUERY, query_idx);

#if ((LWIP_DNS_SECURE & LWIP_DNS_SECURE_RAND_SRC_PORT) != 0)
		pcb_idx = entry->pcb_idx;
#else
		pcb_idx = 0;
#endif
		/* send dns packet */
		LWIP_DEBUGF(DNS_DEBUG, ("sending DNS request ID %d for name \"%s\" to server %d\r\n",
		                        entry->txid, entry->name, entry->server_idx));
#if LWIP_DNS_SUPPORT_MDNS_QUERIES

		if(entry->is_mdns) {
			dst_port = DNS_MQUERY_PORT;
#if LWIP_IPV6

			if(LWIP_DNS_ADDRTYPE_IS_IPV6(entry->reqaddrtype)) {
				dst = &dns_mquery_v6group;
			}

#endif
#if LWIP_IPV4 && LWIP_IPV6
			else
#endif
#if LWIP_IPV4
			{
				dst = &dns_mquery_v4group;
			}

#endif
		} else
#endif /* LWIP_DNS_SUPPORT_MDNS_QUERIES */
		{
			dst_port = DNS_SERVER_PORT;
			dst = &dns_servers[entry->server_idx];
		}

		err = udp_sendto(dns_pcbs[pcb_idx], p, dst, dst_port);

		/* free pbuf */
		pbuf_free(p);
	} else {
		err = ERR_MEM;
	}

	return err;
}

#if ((LWIP_DNS_SECURE & LWIP_DNS_SECURE_RAND_SRC_PORT) != 0)
static struct udp_pcb*
dns_alloc_random_port(void)
{
	err_t err;
	struct udp_pcb* ret;

	ret = udp_new_ip_type(IPADDR_TYPE_ANY);

	if(ret == NULL) {
		/* out of memory, have to reuse an existing pcb */
		return NULL;
	}

	do {
		u16_t port = (u16_t)DNS_RAND_TXID();

		if(!DNS_PORT_ALLOWED(port)) {
			/* this port is not allowed, try again */
			err = ERR_USE;
			continue;
		}

		err = udp_bind(ret, IP_ANY_TYPE, port);
	} while(err == ERR_USE);

	if(err != ERR_OK) {
		udp_remove(ret);
		return NULL;
	}

	udp_recv(ret, dns_recv, NULL);
	return ret;
}

/**
 * dns_alloc_pcb() - allocates a new pcb (or reuses an existing one) to be used
 * for sending a request
 *
 * @return an index into dns_pcbs
 */
static u8_t
dns_alloc_pcb(void)
{
	u8_t i;
	u8_t idx;

	for(i = 0; i < DNS_MAX_SOURCE_PORTS; i++) {
		if(dns_pcbs[i] == NULL) {
			break;
		}
	}

	if(i < DNS_MAX_SOURCE_PORTS) {
		dns_pcbs[i] = dns_alloc_random_port();

		if(dns_pcbs[i] != NULL) {
			/* succeeded */
			dns_last_pcb_idx = i;
			return i;
		}
	}

	/* if we come here, creating a new UDP pcb failed, so we have to use
	   an already existing one */
	for(i = 0, idx = dns_last_pcb_idx + 1; i < DNS_MAX_SOURCE_PORTS; i++, idx++) {
		if(idx >= DNS_MAX_SOURCE_PORTS) {
			idx = 0;
		}

		if(dns_pcbs[idx] != NULL) {
			dns_last_pcb_idx = idx;
			return idx;
		}
	}

	return DNS_MAX_SOURCE_PORTS;
}
#endif /* ((LWIP_DNS_SECURE & LWIP_DNS_SECURE_RAND_SRC_PORT) != 0) */

/**
 * dns_call_found() - call the found callback and check if there are duplicate
 * entries for the given hostname. If there are any, their found callback will
 * be called and they will be removed.
 *
 * @param idx dns table index of the entry that is resolved or removed
 * @param addr IP address for the hostname (or NULL on error or memory shortage)
 */
static void
dns_call_found(u8_t idx, ip_addr_t* addr)
{
#if ((LWIP_DNS_SECURE & (LWIP_DNS_SECURE_NO_MULTIPLE_OUTSTANDING | LWIP_DNS_SECURE_RAND_SRC_PORT)) != 0)
	u8_t i;
#endif

#if LWIP_IPV4 && LWIP_IPV6

	if(addr != NULL) {
		/* check that address type matches the request and adapt the table entry */
		if(IP_IS_V6_VAL(*addr)) {
			LWIP_ASSERT("invalid response", LWIP_DNS_ADDRTYPE_IS_IPV6(dns_table[idx].reqaddrtype));
			dns_table[idx].reqaddrtype = LWIP_DNS_ADDRTYPE_IPV6;
		} else {
			LWIP_ASSERT("invalid response", !LWIP_DNS_ADDRTYPE_IS_IPV6(dns_table[idx].reqaddrtype));
			dns_table[idx].reqaddrtype = LWIP_DNS_ADDRTYPE_IPV4;
		}
	}

#endif /* LWIP_IPV4 && LWIP_IPV6 */

#if ((LWIP_DNS_SECURE & LWIP_DNS_SECURE_NO_MULTIPLE_OUTSTANDING) != 0)

	for(i = 0; i < DNS_MAX_REQUESTS; i++) {
		if(dns_requests[i].found && (dns_requests[i].dns_table_idx == idx)) {
			(*dns_requests[i].found)(dns_table[idx].name, addr, dns_requests[i].arg);
			/* flush this entry */
			dns_requests[i].found = NULL;
		}
	}

#else

	if(dns_requests[idx].found) {
		(*dns_requests[idx].found)(dns_table[idx].name, addr, dns_requests[idx].arg);
	}

	dns_requests[idx].found = NULL;
#endif
#if ((LWIP_DNS_SECURE & LWIP_DNS_SECURE_RAND_SRC_PORT) != 0)

	/* close the pcb used unless other request are using it */
	for(i = 0; i < DNS_MAX_REQUESTS; i++) {
		if(i == idx) {
			continue; /* only check other requests */
		}

		if(dns_table[i].state == DNS_STATE_ASKING) {
			if(dns_table[i].pcb_idx == dns_table[idx].pcb_idx) {
				/* another request is still using the same pcb */
				dns_table[idx].pcb_idx = DNS_MAX_SOURCE_PORTS;
				break;
			}
		}
	}

	if(dns_table[idx].pcb_idx < DNS_MAX_SOURCE_PORTS) {
		/* if we come here, the pcb is not used any more and can be removed */
		udp_remove(dns_pcbs[dns_table[idx].pcb_idx]);
		dns_pcbs[dns_table[idx].pcb_idx] = NULL;
		dns_table[idx].pcb_idx = DNS_MAX_SOURCE_PORTS;
	}

#endif
}

/* Create a query transmission ID that is unique for all outstanding queries */
static u16_t
dns_create_txid(void)
{
	u16_t txid;
	u8_t i;

again:
	txid = (u16_t)DNS_RAND_TXID();

	/* check whether the ID is unique */
	for(i = 0; i < DNS_TABLE_SIZE; i++) {
		if((dns_table[i].state == DNS_STATE_ASKING) &&
		        (dns_table[i].txid == txid)) {
			/* ID already used by another pending query */
			goto again;
		}
	}

	return txid;
}

/**
 * dns_check_entry() - see if entry has not yet been queried and, if so, sends out a query.
 * Check an entry in the dns_table:
 * - send out query for new entries
 * - retry old pending entries on timeout (also with different servers)
 * - remove completed entries from the table if their TTL has expired
 *
 * @param i index of the dns_table entry to check
 */
static void
dns_check_entry(u8_t i)
{
	err_t err;
	struct dns_table_entry* entry = &dns_table[i];

	LWIP_ASSERT("array index out of bounds", i < DNS_TABLE_SIZE);

	switch(entry->state) {
		case DNS_STATE_NEW:
			/* initialize new entry */
			entry->txid = dns_create_txid();
			entry->state = DNS_STATE_ASKING;
			entry->server_idx = 0;
			entry->tmr = 1;
			entry->retries = 0;

			/* send DNS packet for this entry */
			err = dns_send(i);

			if(err != ERR_OK) {
				LWIP_DEBUGF(DNS_DEBUG | LWIP_DBG_LEVEL_WARNING,
				            ("dns_send returned error: %s\n", lwip_strerr(err)));
			}

			break;

		case DNS_STATE_ASKING:
			if(--entry->tmr == 0) {
				if(++entry->retries == DNS_MAX_RETRIES) {
					if((entry->server_idx + 1 < DNS_MAX_SERVERS) && !ip_addr_isany_val(dns_servers[entry->server_idx + 1])
#if LWIP_DNS_SUPPORT_MDNS_QUERIES
					        && !entry->is_mdns
#endif /* LWIP_DNS_SUPPORT_MDNS_QUERIES */
					  ) {
						/* change of server */
						entry->server_idx++;
						entry->tmr = 1;
						entry->retries = 0;
					} else {
						LWIP_DEBUGF(DNS_DEBUG, ("dns_check_entry: \"%s\": timeout\n", entry->name));
						/* call specified callback function if provided */
						dns_call_found(i, NULL);
						/* flush this entry */
						entry->state = DNS_STATE_UNUSED;
						break;
					}
				} else {
					/* wait longer for the next retry */
					entry->tmr = entry->retries;
				}

				/* send DNS packet for this entry */
				err = dns_send(i);

				if(err != ERR_OK) {
					LWIP_DEBUGF(DNS_DEBUG | LWIP_DBG_LEVEL_WARNING,
					            ("dns_send returned error: %s\n", lwip_strerr(err)));
				}
			}

			break;

		case DNS_STATE_DONE:

			/* if the time to live is nul */
			if((entry->ttl == 0) || (--entry->ttl == 0)) {
				LWIP_DEBUGF(DNS_DEBUG, ("dns_check_entry: \"%s\": flush\n", entry->name));
				/* flush this entry, there cannot be any related pending entries in this state */
				entry->state = DNS_STATE_UNUSED;
			}

			break;

		case DNS_STATE_UNUSED:
			/* nothing to do */
			break;

		default:
			LWIP_ASSERT("unknown dns_table entry state:", 0);
			break;
	}
}

/**
 * Call dns_check_entry for each entry in dns_table - check all entries.
 */
static void
dns_check_entries(void)
{
	u8_t i;

	for(i = 0; i < DNS_TABLE_SIZE; ++i) {
		dns_check_entry(i);
	}
}

/**
 * Save TTL and call dns_call_found for correct response.
 */
static void
dns_correct_response(u8_t idx, u32_t ttl)
{
	struct dns_table_entry* entry = &dns_table[idx];

	entry->state = DNS_STATE_DONE;

	LWIP_DEBUGF(DNS_DEBUG, ("dns_recv: \"%s\": response = ", entry->name));
	ip_addr_debug_print(DNS_DEBUG, (&(entry->ipaddr)));
	LWIP_DEBUGF(DNS_DEBUG, ("\n"));

	/* read the answer resource record's TTL, and maximize it if needed */
	entry->ttl = ttl;

	if(entry->ttl > DNS_MAX_TTL) {
		entry->ttl = DNS_MAX_TTL;
	}

	dns_call_found(idx, &entry->ipaddr);

	if(entry->ttl == 0) {
		/* RFC 883, page 29: "Zero values are
		   interpreted to mean that the RR can only be used for the
		   transaction in progress, and should not be cached."
		   -> flush this entry now */
		/* entry reused during callback? */
		if(entry->state == DNS_STATE_DONE) {
			entry->state = DNS_STATE_UNUSED;
		}
	}
}
/**
 * Receive input function for DNS response packets arriving for the dns UDP pcb.
 */
static void
dns_recv(void* arg, struct udp_pcb* pcb, struct pbuf* p, const ip_addr_t* addr, u16_t port)
{
	u8_t i;
	u16_t txid;
	u16_t res_idx;
	struct dns_hdr hdr;
	struct dns_answer ans;
	struct dns_query qry;
	u16_t nquestions, nanswers;

	LWIP_UNUSED_ARG(arg);
	LWIP_UNUSED_ARG(pcb);
	LWIP_UNUSED_ARG(port);

	/* is the dns message big enough ? */
	if(p->tot_len < (SIZEOF_DNS_HDR + SIZEOF_DNS_QUERY)) {
		LWIP_DEBUGF(DNS_DEBUG, ("dns_recv: pbuf too small\n"));
		/* free pbuf and return */
		goto memerr;
	}

	/* copy dns payload inside static buffer for processing */
	if(pbuf_copy_partial(p, &hdr, SIZEOF_DNS_HDR, 0) == SIZEOF_DNS_HDR) {
		/* Match the ID in the DNS header with the name table. */
		txid = lwip_htons(hdr.id);

		for(i = 0; i < DNS_TABLE_SIZE; i++) {
			const struct dns_table_entry* entry = &dns_table[i];

			if((entry->state == DNS_STATE_ASKING) &&
			        (entry->txid == txid)) {

				/* We only care about the question(s) and the answers. The authrr
				   and the extrarr are simply discarded. */
				nquestions = lwip_htons(hdr.numquestions);
				nanswers   = lwip_htons(hdr.numanswers);

				/* Check for correct response. */
				if((hdr.flags1 & DNS_FLAG1_RESPONSE) == 0) {
					LWIP_DEBUGF(DNS_DEBUG, ("dns_recv: \"%s\": not a response\n", entry->name));
					goto memerr; /* ignore this packet */
				}

				if(nquestions != 1) {
					LWIP_DEBUGF(DNS_DEBUG, ("dns_recv: \"%s\": response not match to query\n", entry->name));
					goto memerr; /* ignore this packet */
				}

#if LWIP_DNS_SUPPORT_MDNS_QUERIES

				if(!entry->is_mdns)
#endif /* LWIP_DNS_SUPPORT_MDNS_QUERIES */
				{
					/* Check whether response comes from the same network address to which the
					   question was sent. (RFC 5452) */
					if(!ip_addr_cmp(addr, &dns_servers[entry->server_idx])) {
						goto memerr; /* ignore this packet */
					}
				}

				/* Check if the name in the "question" part match with the name in the entry and
				   skip it if equal. */
				res_idx = dns_compare_name(entry->name, p, SIZEOF_DNS_HDR);

				if(res_idx == 0xFFFF) {
					LWIP_DEBUGF(DNS_DEBUG, ("dns_recv: \"%s\": response not match to query\n", entry->name));
					goto memerr; /* ignore this packet */
				}

				/* check if "question" part matches the request */
				if(pbuf_copy_partial(p, &qry, SIZEOF_DNS_QUERY, res_idx) != SIZEOF_DNS_QUERY) {
					goto memerr; /* ignore this packet */
				}

				if((qry.cls != PP_HTONS(DNS_RRCLASS_IN)) ||
				        (LWIP_DNS_ADDRTYPE_IS_IPV6(entry->reqaddrtype) && (qry.type != PP_HTONS(DNS_RRTYPE_AAAA))) ||
				        (!LWIP_DNS_ADDRTYPE_IS_IPV6(entry->reqaddrtype) && (qry.type != PP_HTONS(DNS_RRTYPE_A)))) {
					LWIP_DEBUGF(DNS_DEBUG, ("dns_recv: \"%s\": response not match to query\n", entry->name));
					goto memerr; /* ignore this packet */
				}

				/* skip the rest of the "question" part */
				res_idx += SIZEOF_DNS_QUERY;

				/* Check for error. If so, call callback to inform. */
				if(hdr.flags2 & DNS_FLAG2_ERR_MASK) {
					LWIP_DEBUGF(DNS_DEBUG, ("dns_recv: \"%s\": error in flags\n", entry->name));
				} else {
					while((nanswers > 0) && (res_idx < p->tot_len)) {
						/* skip answer resource record's host name */
						res_idx = dns_skip_name(p, res_idx);

						if(res_idx == 0xFFFF) {
							goto memerr; /* ignore this packet */
						}

						/* Check for IP address type and Internet class. Others are discarded. */
						if(pbuf_copy_partial(p, &ans, SIZEOF_DNS_ANSWER, res_idx) != SIZEOF_DNS_ANSWER) {
							goto memerr; /* ignore this packet */
						}

						res_idx += SIZEOF_DNS_ANSWER;

						if(ans.cls == PP_HTONS(DNS_RRCLASS_IN)) {
#if LWIP_IPV4

							if((ans.type == PP_HTONS(DNS_RRTYPE_A)) && (ans.len == PP_HTONS(sizeof(ip4_addr_t)))) {
#if LWIP_IPV4 && LWIP_IPV6

								if(!LWIP_DNS_ADDRTYPE_IS_IPV6(entry->reqaddrtype))
#endif /* LWIP_IPV4 && LWIP_IPV6 */
								{
									ip4_addr_t ip4addr;

									/* read the IP address after answer resource record's header */
									if(pbuf_copy_partial(p, &ip4addr, sizeof(ip4_addr_t), res_idx) != sizeof(ip4_addr_t)) {
										goto memerr; /* ignore this packet */
									}

									ip_addr_copy_from_ip4(dns_table[i].ipaddr, ip4addr);
									pbuf_free(p);
									/* handle correct response */
									dns_correct_response(i, lwip_ntohl(ans.ttl));
									return;
								}
							}

#endif /* LWIP_IPV4 */
#if LWIP_IPV6

							if((ans.type == PP_HTONS(DNS_RRTYPE_AAAA)) && (ans.len == PP_HTONS(sizeof(ip6_addr_t)))) {
#if LWIP_IPV4 && LWIP_IPV6

								if(LWIP_DNS_ADDRTYPE_IS_IPV6(entry->reqaddrtype))
#endif /* LWIP_IPV4 && LWIP_IPV6 */
								{
									ip6_addr_t ip6addr;

									/* read the IP address after answer resource record's header */
									if(pbuf_copy_partial(p, &ip6addr, sizeof(ip6_addr_t), res_idx) != sizeof(ip6_addr_t)) {
										goto memerr; /* ignore this packet */
									}

									ip_addr_copy_from_ip6(dns_table[i].ipaddr, ip6addr);
									pbuf_free(p);
									/* handle correct response */
									dns_correct_response(i, lwip_ntohl(ans.ttl));
									return;
								}
							}

#endif /* LWIP_IPV6 */
						}

						/* skip this answer */
						if((int)(res_idx + lwip_htons(ans.len)) > 0xFFFF) {
							goto memerr; /* ignore this packet */
						}

						res_idx += lwip_htons(ans.len);
						--nanswers;
					}

#if LWIP_IPV4 && LWIP_IPV6

					if((entry->reqaddrtype == LWIP_DNS_ADDRTYPE_IPV4_IPV6) ||
					        (entry->reqaddrtype == LWIP_DNS_ADDRTYPE_IPV6_IPV4)) {
						if(entry->reqaddrtype == LWIP_DNS_ADDRTYPE_IPV4_IPV6) {
							/* IPv4 failed, try IPv6 */
							dns_table[i].reqaddrtype = LWIP_DNS_ADDRTYPE_IPV6;
						} else {
							/* IPv6 failed, try IPv4 */
							dns_table[i].reqaddrtype = LWIP_DNS_ADDRTYPE_IPV4;
						}

						pbuf_free(p);
						dns_table[i].state = DNS_STATE_NEW;
						dns_check_entry(i);
						return;
					}

#endif /* LWIP_IPV4 && LWIP_IPV6 */
					LWIP_DEBUGF(DNS_DEBUG, ("dns_recv: \"%s\": error in response\n", entry->name));
				}

				/* call callback to indicate error, clean up memory and return */
				pbuf_free(p);
				dns_call_found(i, NULL);
				dns_table[i].state = DNS_STATE_UNUSED;
				return;
			}
		}
	}

memerr:
	/* deallocate memory and return */
	pbuf_free(p);
	return;
}

/**
 * Queues a new hostname to resolve and sends out a DNS query for that hostname
 *
 * @param name the hostname that is to be queried
 * @param hostnamelen length of the hostname
 * @param found a callback function to be called on success, failure or timeout
 * @param callback_arg argument to pass to the callback function
 * @return err_t return code.
 */
static err_t
dns_enqueue(const char* name, size_t hostnamelen, dns_found_callback found,
            void* callback_arg LWIP_DNS_ADDRTYPE_ARG(u8_t dns_addrtype) LWIP_DNS_ISMDNS_ARG(u8_t is_mdns))
{
	u8_t i;
	u8_t lseq, lseqi;
	struct dns_table_entry* entry = NULL;
	size_t namelen;
	struct dns_req_entry* req;

#if ((LWIP_DNS_SECURE & LWIP_DNS_SECURE_NO_MULTIPLE_OUTSTANDING) != 0)
	u8_t r;

	/* check for duplicate entries */
	for(i = 0; i < DNS_TABLE_SIZE; i++) {
		if((dns_table[i].state == DNS_STATE_ASKING) &&
		        (lwip_strnicmp(name, dns_table[i].name, sizeof(dns_table[i].name)) == 0)) {
#if LWIP_IPV4 && LWIP_IPV6

			if(dns_table[i].reqaddrtype != dns_addrtype) {
				/* requested address types don't match
				   this can lead to 2 concurrent requests, but mixing the address types
				   for the same host should not be that common */
				continue;
			}

#endif /* LWIP_IPV4 && LWIP_IPV6 */

			/* this is a duplicate entry, find a free request entry */
			for(r = 0; r < DNS_MAX_REQUESTS; r++) {
				if(dns_requests[r].found == 0) {
					dns_requests[r].found = found;
					dns_requests[r].arg = callback_arg;
					dns_requests[r].dns_table_idx = i;
					LWIP_DNS_SET_ADDRTYPE(dns_requests[r].reqaddrtype, dns_addrtype);
					LWIP_DEBUGF(DNS_DEBUG, ("dns_enqueue: \"%s\": duplicate request\n", name));
					return ERR_INPROGRESS;
				}
			}
		}
	}

	/* no duplicate entries found */
#endif

	/* search an unused entry, or the oldest one */
	lseq = 0;
	lseqi = DNS_TABLE_SIZE;

	for(i = 0; i < DNS_TABLE_SIZE; ++i) {
		entry = &dns_table[i];

		/* is it an unused entry ? */
		if(entry->state == DNS_STATE_UNUSED) {
			break;
		}

		/* check if this is the oldest completed entry */
		if(entry->state == DNS_STATE_DONE) {
			u8_t age = dns_seqno - entry->seqno;

			if(age > lseq) {
				lseq = age;
				lseqi = i;
			}
		}
	}

	/* if we don't have found an unused entry, use the oldest completed one */
	if(i == DNS_TABLE_SIZE) {
		if((lseqi >= DNS_TABLE_SIZE) || (dns_table[lseqi].state != DNS_STATE_DONE)) {
			/* no entry can be used now, table is full */
			LWIP_DEBUGF(DNS_DEBUG, ("dns_enqueue: \"%s\": DNS entries table is full\n", name));
			return ERR_MEM;
		} else {
			/* use the oldest completed one */
			i = lseqi;
			entry = &dns_table[i];
		}
	}

#if ((LWIP_DNS_SECURE & LWIP_DNS_SECURE_NO_MULTIPLE_OUTSTANDING) != 0)
	/* find a free request entry */
	req = NULL;

	for(r = 0; r < DNS_MAX_REQUESTS; r++) {
		if(dns_requests[r].found == NULL) {
			req = &dns_requests[r];
			break;
		}
	}

	if(req == NULL) {
		/* no request entry can be used now, table is full */
		LWIP_DEBUGF(DNS_DEBUG, ("dns_enqueue: \"%s\": DNS request entries table is full\n", name));
		return ERR_MEM;
	}

	req->dns_table_idx = i;
#else
	/* in this configuration, the entry index is the same as the request index */
	req = &dns_requests[i];
#endif

	/* use this entry */
	LWIP_DEBUGF(DNS_DEBUG, ("dns_enqueue: \"%s\": use DNS entry %"U16_F"\n", name, (u16_t)(i)));

	/* fill the entry */
	entry->state = DNS_STATE_NEW;
	entry->seqno = dns_seqno;
	LWIP_DNS_SET_ADDRTYPE(entry->reqaddrtype, dns_addrtype);
	LWIP_DNS_SET_ADDRTYPE(req->reqaddrtype, dns_addrtype);
	req->found = found;
	req->arg   = callback_arg;
	namelen = LWIP_MIN(hostnamelen, DNS_MAX_NAME_LENGTH - 1);
	MEMCPY(entry->name, name, namelen);
	entry->name[namelen] = 0;

#if ((LWIP_DNS_SECURE & LWIP_DNS_SECURE_RAND_SRC_PORT) != 0)
	entry->pcb_idx = dns_alloc_pcb();

	if(entry->pcb_idx >= DNS_MAX_SOURCE_PORTS) {
		/* failed to get a UDP pcb */
		LWIP_DEBUGF(DNS_DEBUG, ("dns_enqueue: \"%s\": failed to allocate a pcb\n", name));
		entry->state = DNS_STATE_UNUSED;
		req->found = NULL;
		return ERR_MEM;
	}

	LWIP_DEBUGF(DNS_DEBUG, ("dns_enqueue: \"%s\": use DNS pcb %"U16_F"\n", name, (u16_t)(entry->pcb_idx)));
#endif

#if LWIP_DNS_SUPPORT_MDNS_QUERIES
	entry->is_mdns = is_mdns;
#endif

	dns_seqno++;

	/* force to send query without waiting timer */
	dns_check_entry(i);

	/* dns query is enqueued */
	return ERR_INPROGRESS;
}

/**
 * @ingroup dns
 * Resolve a hostname (string) into an IP address.
 * NON-BLOCKING callback version for use with raw API!!!
 *
 * Returns immediately with one of err_t return codes:
 * - ERR_OK if hostname is a valid IP address string or the host
 *   name is already in the local names table.
 * - ERR_INPROGRESS enqueue a request to be sent to the DNS server
 *   for resolution if no errors are present.
 * - ERR_ARG: dns client not initialized or invalid hostname
 *
 * @param hostname the hostname that is to be queried
 * @param addr pointer to a ip_addr_t where to store the address if it is already
 *             cached in the dns_table (only valid if ERR_OK is returned!)
 * @param found a callback function to be called on success, failure or timeout (only if
 *              ERR_INPROGRESS is returned!)
 * @param callback_arg argument to pass to the callback function
 * @return a err_t return code.
 */
err_t
dns_gethostbyname(const char* hostname, ip_addr_t* addr, dns_found_callback found,
                  void* callback_arg)
{
	return dns_gethostbyname_addrtype(hostname, addr, found, callback_arg, LWIP_DNS_ADDRTYPE_DEFAULT);
}

/**
 * @ingroup dns
 * Like dns_gethostbyname, but returned address type can be controlled:
 * @param hostname the hostname that is to be queried
 * @param addr pointer to a ip_addr_t where to store the address if it is already
 *             cached in the dns_table (only valid if ERR_OK is returned!)
 * @param found a callback function to be called on success, failure or timeout (only if
 *              ERR_INPROGRESS is returned!)
 * @param callback_arg argument to pass to the callback function
 * @param dns_addrtype - LWIP_DNS_ADDRTYPE_IPV4_IPV6: try to resolve IPv4 first, try IPv6 if IPv4 fails only
 *                     - LWIP_DNS_ADDRTYPE_IPV6_IPV4: try to resolve IPv6 first, try IPv4 if IPv6 fails only
 *                     - LWIP_DNS_ADDRTYPE_IPV4: try to resolve IPv4 only
 *                     - LWIP_DNS_ADDRTYPE_IPV6: try to resolve IPv6 only
 */
err_t
dns_gethostbyname_addrtype(const char* hostname, ip_addr_t* addr, dns_found_callback found,
                           void* callback_arg, u8_t dns_addrtype)
{
	size_t hostnamelen;
#if LWIP_DNS_SUPPORT_MDNS_QUERIES
	u8_t is_mdns;
#endif

	/* not initialized or no valid server yet, or invalid addr pointer
	 * or invalid hostname or invalid hostname length */
	if((addr == NULL) ||
	        (!hostname) || (!hostname[0])) {
		return ERR_ARG;
	}

#if ((LWIP_DNS_SECURE & LWIP_DNS_SECURE_RAND_SRC_PORT) == 0)

	if(dns_pcbs[0] == NULL) {
		return ERR_ARG;
	}

#endif
	hostnamelen = strlen(hostname);

	if(hostnamelen >= DNS_MAX_NAME_LENGTH) {
		LWIP_DEBUGF(DNS_DEBUG, ("dns_gethostbyname: name too long to resolve"));
		return ERR_ARG;
	}


#if LWIP_HAVE_LOOPIF

	if(strcmp(hostname, "localhost") == 0) {
		ip_addr_set_loopback(LWIP_DNS_ADDRTYPE_IS_IPV6(dns_addrtype), addr);
		return ERR_OK;
	}

#endif /* LWIP_HAVE_LOOPIF */

	/* host name already in octet notation? set ip addr and return ERR_OK */
	if(ipaddr_aton(hostname, addr)) {
#if LWIP_IPV4 && LWIP_IPV6

		if((IP_IS_V6(addr) && (dns_addrtype != LWIP_DNS_ADDRTYPE_IPV4)) ||
		        (IP_IS_V4(addr) && (dns_addrtype != LWIP_DNS_ADDRTYPE_IPV6)))
#endif /* LWIP_IPV4 && LWIP_IPV6 */
		{
			return ERR_OK;
		}
	}

	/* already have this address cached? */
	if(dns_lookup(hostname, addr LWIP_DNS_ADDRTYPE_ARG(dns_addrtype)) == ERR_OK) {
		return ERR_OK;
	}

#if LWIP_IPV4 && LWIP_IPV6

	if((dns_addrtype == LWIP_DNS_ADDRTYPE_IPV4_IPV6) || (dns_addrtype == LWIP_DNS_ADDRTYPE_IPV6_IPV4)) {
		/* fallback to 2nd IP type and try again to lookup */
		u8_t fallback;

		if(dns_addrtype == LWIP_DNS_ADDRTYPE_IPV4_IPV6) {
			fallback = LWIP_DNS_ADDRTYPE_IPV6;
		} else {
			fallback = LWIP_DNS_ADDRTYPE_IPV4;
		}

		if(dns_lookup(hostname, addr LWIP_DNS_ADDRTYPE_ARG(fallback)) == ERR_OK) {
			return ERR_OK;
		}
	}

#else /* LWIP_IPV4 && LWIP_IPV6 */
	LWIP_UNUSED_ARG(dns_addrtype);
#endif /* LWIP_IPV4 && LWIP_IPV6 */

#if LWIP_DNS_SUPPORT_MDNS_QUERIES

	if(strstr(hostname, ".local") == &hostname[hostnamelen] - 6) {
		is_mdns = 1;
	} else {
		is_mdns = 0;
	}

	if(!is_mdns)
#endif /* LWIP_DNS_SUPPORT_MDNS_QUERIES */
	{
		/* prevent calling found callback if no server is set, return error instead */
		if(ip_addr_isany_val(dns_servers[0])) {
			return ERR_VAL;
		}
	}

	/* queue query with specified callback */
	return dns_enqueue(hostname, hostnamelen, found, callback_arg LWIP_DNS_ADDRTYPE_ARG(dns_addrtype)
	                   LWIP_DNS_ISMDNS_ARG(is_mdns));
}

#endif /* LWIP_DNS */
